A boundary microphone is a kind of microphone of a flat, low profile shape for use on a table, floor, or the like, and includes a base plate 10 and a cover 20 closing the base plate 10 as shown in FIG. 4.
The base plate 10, whose base surface 10a is flat, includes a recess 11 for mounting a circuit board and a column 12 for connecting the cover on the upper surface side, and is provided with a backwall 13 that has a microphone cable drawing hole 13a in the rear. In many cases, the base plate 10 as a whole is made of die-cast aluminum, for example.
Although not shown in detail, a punched plate (perforated metal) or a wire mesh material, which has a number of sound wave through holes, is used for the cover 20. The cover 20 is screwed down to the column 12 by a male screw 21 to form an inner space on the upper surface side of the base plate 10.
A circuit board 30 that has a sound signal output part 31 is mounted over the recess 11 of the base plate 10. Although not shown, the sound signal output part 31 includes an impedance converter such as a field effect transistor (FET), a tone control circuit, a filter circuit, and the like.
The circuit board 30 is provided with an opening 32 in a part of the circuit board, and a microphone unit 40 is disposed in the opening 32 with a holder 50. Typically, a unidirectional condenser microphone unit that has a front acoustic terminal 40a and a rear acoustic terminal 40b is used for the microphone unit 40.
The microphone cable drawing hole 13a on the backwall 13 is provided with a code bushing 14, through which a microphone cable 15 is drawn into the inner space and electrically connected to a terminal portion on the circuit board 30.
The base plate 10 and the cover 20 are in contact with each other and in a conductive state in terms of DC; however, they suffer from insufficient electrostatic shielding against considerably strong electromagnetic waves radiated from a mobile phone.
Therefore, when the mobile phone is used near the boundary microphone, a high frequency current caused by the electromagnetic waves may flow in the microphone and may be detected by a semiconductor device such as the FET mounted on the circuit board 30, causing a noise.
Therefore, the inventor has proposed in Japanese Patent No. 4471818 to orient the side of a component mounting surface 30b of the circuit board 30, on which the sound signal output part 31 is located, toward the recess 11 side. A shield pattern composed of a solid pattern of copper foil is formed on the upper surface (opposite to the component mounting surface) 30a side of the circuit board 30 so that the shield pattern and the base plate 10 together form a shield case for the recess 11.
Although this reduces noises caused by external electromagnetic waves to some extent, there still remain some problems to be solved.
Referring to FIG. 4, the microphone unit 40 is fitted and held in the holder 50 that, on the rear end side, consists of a metallic cylinder with a base and disposed in the opening 32 of the circuit board with the holder 50. Conventionally, lead wires 42, 42 attached to a terminal board 41 of the microphone unit 40 are drawn through a small hole 51 in the base surface of the holder 50 and soldered to a predetermined land section of the circuit board 30.
This configuration suffers from insufficient electrostatic shielding on the portions of the lead wires 42, 42 drawn out of the holder 50 and extending to the circuit board 30, and when electromagnetic waves are applied to the portions of lead wires, a high frequency current may flow in the recess 11, still causing a noise.
In addition, since the lead wires 42, 42 are of coated type, which makes it difficult to uniformly maintain the shape of wires, another problem is that frequencies of electromagnetic waves that cause noises and the levels of the noises are non-uniform.
Therefore, it is an object of the invention to improve a boundary microphone described in Japanese Patent No. 4471818 to effectively reduce noises caused by external electromagnetic waves by providing sufficient electrostatic shielding, including on the portions of lead wires from a microphone unit to a circuit board.